Process for preparing MMA in high yields

Abstract

A process for preparing methyl methacrylate by direct oxidative esterification of methacrolein has elevated yields compared to known processes. Methyl methacrylate (MMA) is used in large amounts for preparing polymers and copolymers with other polymerizable compounds. In addition, methyl methacrylate is an important synthesis unit for a variety of specialty esters based on methacrylic acid (MAA), which can be produced by transesterification with the appropriate alcohol. There is consequently a great interest in very simple, economic, and environmentally friendly processes for preparing methyl methacrylate. A superior workup of the reactor output from the oxidative esterification of methacrolein allows specific by-products to be isolated and then additionally converted to alkyl methacrylates, especially to MMA.

The invention claimed is: 
     
       1. A process for preparing alkyl methacrylates, comprising:
 preparing methacrolein in a first reaction stage in a reactor I, and 
 oxidatively esterifying the methacrolein with an alcohol in a second reaction stage in a reactor II to give an alkyl methacrylate, wherein 
 a. a reactor output from reactor II is separated into a first fraction containing a predominant portion of the alkyl methacrylate and a second fraction containing methacrylic acid and an alkyl alkoxyisobutyrate, and 
 b. the second fraction is converted in a reactor III in such a way that further alkyl methacrylate is formed from the alkyl alkoxyisobutyrate and the methacrylic acid. 
 
     
     
       2. The process according to  claim 1 , wherein the separation in step a is effected by at least an extraction and/or a distillation. 
     
     
       3. The process according to  claim 1 , wherein the reaction in step b is conducted at a temperature equal to or higher than a temperature of the oxidative reaction in reactor II. 
     
     
       4. The process according to  claim 1 , wherein the reaction in step b is conducted with a reaction mixture comprising the alkyl alkoxyisobutyrate, the methacrylic acid, dimeric methacrolein (DIMAL) as a by-product from reactor II, one or more derivatives of dimeric methacrolein as a by-product from reactor II, water, and free alcohol,
 wherein the free alcohol is optionally added from a separate feed. 
 
     
     
       5. The process according to  claim 1 , wherein the second fraction is reacted in reactor III at a temperature of at least 90° C. 
     
     
       6. The process according to  claim 1 , wherein the second fraction is reacted in reactor III in step b in the presence of a catalyst. 
     
     
       7. The process according to  claim 1 , wherein step a comprises freeing the reactor output from reactor II of methacrolein and partly of the alcohol in a first distillation column, to obtain a stream comprising:
 alkyl methacrylate, 
 water, 
 methacrylic acid and/or an alkali metal methacrylate obtained from at least partial neutralization of methacrylic acid with an alkaline or basic auxiliary, 
 alkyl alkoxyisobutyrate, and 
 alcohol, 
 wherein step a then further comprises admixing the stream with a strong acid and separating in an extraction into a hydrophobic phase comprising alkyl methacrylate, a greater fraction of methacrylic acid and alkyl alkoxyisobutyrate, and a hydrophilic phase comprising water, the alcohol, and fractions of alkyl methacrylate and methacrylic acid, and 
 wherein step a then further comprises separation of the hydrophobic phase into the first fraction containing the predominant portion of the alkyl methacrylate and the second fraction containing the methacrylic acid and the alkyl alkoxyisobutyrate. 
 
     
     
       8. The process according to  claim 1 , wherein step a comprises freeing the reactor output from reactor II of methacrolein and partly of the alcohol in a first distillation column, to obtain a stream comprising:
 alkyl methacrylate, 
 water, 
 methacrylic acid and/or an alkali metal methacrylate obtained from at least partial neutralization of methacrylic acid with an alkaline or basic auxiliary, 
 alkyl alkoxyisobutyrate, and 
 alcohol, 
 wherein the stream is then separated in a second distillation into a light phase comprising alkyl methacrylate and the alcohol as the first phase, and a heavy phase comprising water, alkyl alkoxyisobutyrate and methacrylic acid and/or the alkali metal methacrylate, as the second phase. 
 
     
     
       9. The process according to  claim 1 , wherein the alcohol is methanol, the alkyl methacrylate is methyl methacrylate and the alkyl alkoxyisobutyrate is methyl methoxyisobutyrate (MMIB). 
     
     
       10. The process according to  claim 1 , wherein the reaction in reactor III is effected at a temperature between 80 and 170° C. 
     
     
       11. The process according to  claim 6 , wherein the catalyst in reactor III is sulfuric acid. 
     
     
       12. The process according to  claim 1 , wherein the first reaction stage in reactor I is a reaction of propanal with formalin. 
     
     
       13. The process according to  claim 1 , wherein the first reaction stage in reactor I is a reaction of isobutene and/or tert-butanol with atmospheric oxygen in the presence of a heterogeneous catalyst at temperatures of 300 to 500° C. to form methacrolein, wherein the methacrolein is condensed and worked up to a purity of at least 80% and isolated in liquid form, and wherein the methacrolein is then sent to the further reaction in reactor II of oxidative esterification. 
     
     
       14. The process according to  claim 4 , wherein step a comprises freeing the reactor output from reactor II of methacrolein and partly of the alcohol in a first distillation column, to obtain a stream comprising:
 alkyl methacrylate, 
 water, 
 methacrylic acid and/or an alkali metal methacrylate obtained from at least partial neutralization of methacrylic acid with an alkaline or basic auxiliary, 
 alkyl alkoxyisobutyrate, and 
 alcohol, 
 wherein the stream is then separated in a second distillation into a light phase comprising alkyl methacrylate and the alcohol as the first phase, and a heavy phase comprising water, alkyl alkoxyisobutyrate, methacrylic acid and/or the alkali metal methacrylate, dimeric methacrolein and an alkyl ester derivative of dimeric methacrolein, as the second phase. 
 
     
     
       15. The process according to  claim 14 , wherein dimeric methacrolein is cleaved in reactor III into methacrolein, and the alkyl ester derivative of dimeric methacrolein is cleaved into methacrolein and the alkyl methacrylate. 
     
     
       16. The process according to  claim 15 , wherein the methacrolein from reactor III is separated from the alkyl methacrylate in a later distillation stage and returned to reactor II. 
     
     
       17. The process according to  claim 16 , wherein the heavy phase further comprises terephthalic acid obtained as a by-product, and wherein the terephthalic acid is removed from a reactor output from reactor III as a high-boiling component by distillation or as a hydrophilic component by extraction. 
     
     
       18. The process according to  claim 6 , wherein the catalyst is a Brønsted acid and wherein, after the second fraction is reacted in reactor III, the catalyst is recycled into reactor III or another workup step. 
     
     
       19. The process according to  claim 4 , wherein the derivative of dimeric methacrolein is a dimeric methacrolein ester. 
     
     
       20. The process according to  claim 6 , wherein the second fraction is reacted in reactor III in the presence of a Brønsted acid.